The Republican chairmen of two US Senate committees have asked the Treasury Department to provide any reports of money laundering or fraud related to former Vice President Joe Biden's son's business dealings with a Ukraine energy firm, according to a letter seen by Reuters on Friday.

The letter seeks "suspicious activity reports," which are documents that financial institutions file with the Treasury Department's Financial Crimes Enforcement Network whenever there is a suspected case of money laundering or fraud. It was unclear whether any such reports exist related to Hunter Biden, the former vice president's son.

The request comes as Republicans seek to defend President Donald Trump against a Democrat-led impeachment probe into whether the president improperly pressured Ukraine to investigate the Bidens to improve his reelection chances.

Senate Finance Committee Chairman Chuck Grassley and Homeland Security and Governmental Affairs Committee Chairman Ron Johnson sent the request in a letter dated Nov. 15 to Treasury Department Director of Financial Crimes Enforcement Network Ken Blanco.

The chairmen said they want information by Dec. 5 related to Hunter Biden, who was on the board of directors of Burisma Holdings, which had been under investigation in Ukraine.

In their letter the senators noted Burisma was paying Hunter Biden as much as $50,000 a month and that their panels are investigating "potentially improper actions by the Obama administration with respect to Burisma Holdings and Ukraine."

Grassley and Johnson, citing a story by a reporter with conservative ties, said Burisma's consulting firm Blue Star Strategies used Biden's board membership to gain access to Obama administration officials at the State Department.

The Bidens have denied any wrongdoing.

The elder Biden is a leading Democratic candidate for president in next year's U.S. elections in which Trump is seeking a second four-year term.

Trump and Republicans in Congress have been ramping up their rhetoric on the Bidens as the November 2020 U.S. elections near and as Democrats in the House of Representatives intensify their impeachment investigation of Trump.

Democrats are looking into whether Trump used the withholding of U.S. aid to Ukraine as leverage to press Kiev to launch investigations into the Bidens and allegations Ukraine meddled in the 2016 U.S. elections to hurt the Trump campaign. U.S. intelligence agencies have concluded it was Russia that tried to influence the 2016 election in favor of Trump.

On Thursday, Senate Judiciary Committee Chairman Lindsey Graham, an ally of Trump's, wrote to Secretary of State Mike Pompeo requesting documents related to 2016 contacts between the Bidens, other Obama administration officials and former Ukrainian President Petro Poroshenko.

Trump has denied doing anything improper in Ukraine and has called the impeachment inquiry a witch hunt.